Method of crimping a polymeric stent

ABSTRACT

A method of crimping a stent to a support element is disclosed, the method comprising: positioning a polymeric stent around a support element; heating the stent, wherein the heated stent is above ambient temperature; and allowing the heated stent to radially contract onto the support element, wherein the heated stent radially contracts at least partially due to heating the stent.

CROSS-REFERENCE

This application is a continuation application of U.S. application Ser.No. 11/441,996, filed on May 25, 2006 now U.S. Pat. No. 7,761,968 B2,which is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods of crimping a stent onto aballoon-catheter assembly to increase retention of the stent thereon.

2. Description of the State of the Art

This invention relates to radially expandable endoprostheses, which areadapted to be implanted in a bodily lumen. An “endoprosthesis”corresponds to an artificial device that is placed inside the body. A“lumen” refers to a cavity of a tubular organ such as a blood vessel.

A stent is an example of such an endoprosthesis. Stents are generallycylindrically shaped devices, which function to hold open and sometimesexpand a segment of a blood vessel or other anatomical lumen such asurinary tracts and bile ducts. Stents are often used in the treatment ofatherosclerotic stenosis in blood vessels. “Stenosis” refers to anarrowing or constriction of the diameter of a bodily passage ororifice. In such treatments, stents reinforce body vessels and preventrestenosis following angioplasty. “Restenosis” refers to thereoccurrence of stenosis in a blood vessel or heart valve after it hasbeen subjected to angioplasty or valvuloplasty.

The stent must be able to satisfy a number of mechanical requirements.First, the stent must be capable of withstanding the structural loads,namely radial compressive forces, imposed on the stent as it supportsthe walls of a vessel. Therefore, a stent must possess adequate radialstrength. Radial strength, which is the ability of a stent to resistradial compressive forces, is due to strength and rigidity around acircumferential direction of the stent. Radial strength and rigidity,therefore, may also be described as, hoop or circumferential strengthand rigidity. Once expanded, the stent must adequately maintain its sizeand shape throughout its service life despite the various forces thatmay come to bear on it, including the cyclic loading induced by thebeating heart.

A stent is typically composed of scaffolding that includes a pattern ornetwork of interconnecting structural elements often referred to in theart as struts or bar arms. The scaffolding can be formed from wires,tubes, or sheets of material rolled into a cylindrical shape. Thescaffolding is designed so that the stent can be radially compressed toallow crimping and radially expanded to allow deployment, as describedbelow.

Additionally, it may be desirable for a stent to be biodegradable. Inmany treatment applications, the presence of a stent in a body may benecessary for a limited period of time until its intended function of,for example, maintaining vascular patency and/or drug delivery isaccomplished. Thus, stents are often fabricated from biodegradable,bioabsorbable, and/or bioerodable materials such that they completelyerode only after the clinical need for them has ended.

In general, there are several important aspects in the mechanicalbehavior of polymers that affect stent design. Polymers tend to havelower strength than metals on a per unit mass basis. Polymeric struts orbar arms in a polymeric stent can crack during crimping and expansion,especially when brittle polymers are used. The portions of the stentsubjected to substantial deformation tend to be the most vulnerable tofailure. Furthermore, in order to have adequate mechanical strength,polymeric stents may require significantly thicker struts than requiredin metallic stents, which results in undesirably wider struts inpolymeric stents.

Delivery and deployment of a stent are accomplished by positioning thestent about one end of a catheter, inserting the end of the catheterthrough the skin into a bodily lumen, advancing the catheter in thebodily lumen to a desired treatment location, inflating the stent at thetreatment location, and removing the catheter from the lumen bydeflating the balloon. “Delivery” refers to introducing and transportingthe stent through a bodily lumen to the treated site in a vessel.“Deployment” corresponds to expanding the stent within the lumen at thetreatment site. In the case of a balloon expandable stent, the stent ismounted about a balloon disposed on the catheter. Mounting the stenttypically involves compressing or crimping the stent onto theballoon-catheter assembly. The stent must have sufficient retentionduring delivery prior to deployment of the stent at an implant site.Once the stent and catheter is positioned at an implant site, the stentis then expanded by inflating the balloon.

The balloon-catheter assembly and the attached stent must have a smalldelivery diameter to be able to be transported through the smalldiameter of the vascular system. Also, the stent must be firmly attachedto the catheter to avoid detachment of the stent before it is deliveredand deployed in the lumen of the patient. Detachment of the stent canresult in medical complications. For example, a detached stent can actas an embolus that may create thrombosis and require surgicalintervention. For this reason, stent retention on the balloon-catheterassembly is important.

What is needed is a method for crimping a polymeric stent to increasestent retention on a delivery balloon, while maintaining mechanicalproperties of the stent during crimping.

SUMMARY

Various embodiments of the present invention include a method forfabricating a medical assembly comprising: providing a polymeric tubefabricated from a polymer of an original inside diameter; radiallyexpanding the polymeric tube; fabricating a stent from the radiallyexpanded polymeric tube; positioning the stent around a support element;heating the stent, wherein the heated stent is above ambienttemperature; and allowing the heated stent to radially contract onto thesupport element, wherein the heated stent radially contracts at leastpartially due to heat shrinking of the polymer of the stent, and whereinthe inside diameter of the contracted stent is greater than the originalinside diameter of the polymeric tube.

Further embodiments of the present invention include a method forfabricating a medical assembly comprising: providing a polymeric tubefabricated from a polymer of an original inside diameter; radiallyexpanding the polymeric tube; fabricating a stent from the radiallyexpanded polymeric tube; positioning the stent around a support element;heating the stent; and applying inward radial pressure on the heatedstent to facilitate radial contraction of the stent, wherein the insidediameter of the contracted stent is greater than the original insidediameter of the polymeric tube.

Additional embodiments of the present invention include a method forfabricating a medical assembly comprising: providing a polymeric tubefabricated from a polymer of an original inside diameter, wherein thepolymer has a glass transition temperature; radially expanding thepolymeric tube; fabricating a stent from the radially expanded polymerictube; positioning the stent around a support element; heating the stent,wherein the heated stent is above ambient temperature and at atemperature from a glass transition temperature to a melting temperatureof the polymer; and allowing the heated stent to radially contract ontothe support element, wherein the heated stent radially contracts atleast partially due to heat shrinking of the polymer of the stent, andwherein the inside diameter of the contracted stent is greater than theoriginal inside diameter of the polymeric tube.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a stent.

FIG. 2 depicts a polymeric tube.

FIG. 3( a) depicts a polymeric tube that is radially expanded from anoriginal inside diameter to a radially expanded inside diameter.

FIG. 3( b) depicts a stent fabricated from the polymeric tube that heatshrinks from a radially expanded diameter.

FIG. 4( a) depicts a side view of balloon-catheter assembly.

FIG. 4( b) depicts a side view of a radially expanded stent that ispositioned adjacent the balloon.

FIG. 4( c) depicts a side view of a stent that has been allowed to heatshrink onto the balloon.

DETAILED DESCRIPTION

For the purposes of the present invention, the following terms anddefinitions apply:

The “glass transition temperature,” Tg, of a polymer is the temperatureat which the polymer's amorphous domains transform from a brittlevitreous state to a solid deformable or ductile state at atmosphericpressure. In other words, Tg corresponds to the temperature wheresegmental motion starts in the polymer chains. When an amorphous orsemicrystalline polymer is exposed to an increasing temperature, boththe polymer's coefficient of expansion and heat capacity increase as thetemperature is raised, indicating increased molecular motion. As thetemperature is raised, the actual molecular volume in the sample remainsconstant, and so a higher coefficient of expansion points to an increasein free volume associated with the system and therefore increasedfreedom for the molecules to move. The increasing heat capacitycorresponds to an increase in heat dissipation through movement. Tg of agiven polymer can be dependent on the heating rate and can be influencedby the thermal history of the polymer. Furthermore, the chemicalstructure of the polymer heavily influences the glass transition byaffecting mobility.

“Use” of a stent includes manufacturing, assembling (e.g., crimping astent on balloon), delivery of a stent through a bodily lumen to atreatment site, deployment of a stent at a treatment site, and treatmentof a deployed stent. Both a scaffolding or substrate and a coating on ascaffolding experience stress. For example, during deployment, thescaffolding and/or coating of a stent can be exposed to stress caused bythe radial expansion of the stent body. In addition, the scaffoldingand/or coating may be exposed to stress when it is mounted on a catheterfrom crimping or compression of the stent. These stresses can cause thescaffolding and/or coating to fracture and the coating to tear and/ordetach from the scaffolding. Failure of the mechanical integrity of thestent while the stent is in a patient can lead to serious consequences.For example, there is a risk of embolization caused by pieces of thepolymeric scaffolding and/or coating breaking off from the stent.

FIG. 1 depicts an example of a three-dimensional view of a stent 100.Stent 100 includes a pattern with a number of interconnecting structuralelements or struts 110. The embodiments disclosed herein are not limitedto stents or to the stent pattern illustrated in FIG. 1, as other stentsand patterns are possible. A pattern can be formed on a tube by, forexample, laser cutting a pattern on the tube.

A stent may be fabricated from a polymeric conduit or tube, such aspolymeric tube 200 depicted in FIG. 2. Polymeric tube 200 may becylindrical or substantially cylindrical in shape. Polymeric tube 200has an outside diameter 210 and an inside diameter 220. Tube 200 has asurface 230 and a cylindrical axis 240.

In general, a stent can be capable of withstanding the structural loads,namely radial compressive forces, imposed on the stent as it supportsthe walls of a vessel. Therefore, a stent must possess adequate radialstrength. Radial strength, which is the ability of a stent to resistradial compressive forces, is due to strength and rigidity around acircumferential direction of the stent. Additionally, a stent shouldalso have adequate strength along the axial direction. Therefore, it isdesirable to fabricate a stent from polymeric tube 200 with adequatestrength in the axial direction, as shown by an arrow 250 and in thecircumferential direction as indicated by an arrow 260. A stentfabricated from a tube with biaxial molecular orientation, in otherwords, a tube with a desired degree of polymer chain alignment in boththe axial and the circumferential directions, exhibits better mechanicalbehavior during use of the stent.

Stent retention on the balloon-catheter assembly be difficult forpolymeric stents. One reason for stent retention being more difficult inpolymeric stents is that polymeric stents may require relatively widerstruts than metal stents to achieve required mechanical properties.Wider struts in polymeric stents result in smaller gaps between struts,providing less space for the balloon to protrude through when the stentis crimped onto the balloon. Protrusion of the balloon through stentgaps greatly facilitates stent retention. The reduced stent retentiondue to wider struts in polymer stents increases the likelihood ofdetachment of the stent or premature deployment of a stent in the body

Furthermore, the polymeric delivery balloon is often heated duringcrimping to enhance or facilitate protrusion of the balloon through gapsbetween stent struts. However, Tg of the balloon can be similar to Tg ofthe stent. Since heating the balloon also heats the stent, the stentbecomes more flexible which further reduces protrusion of the balloonthrough stent struts.

As is known by those of skill in the art, applying stress to deform apolymeric material can increase the strength of the material along thedirection of stress. The induced strength arises from alignment ofpolymer chains along the direction of stress. Thus, radial strength in apolymeric tube can be induced by radially expanding a polymeric tube.Additionally, axial strength can be induced by axially deforming a tube.The increased strength allows fabrication of a stent with thinner strutswhich enhances protrusion of a balloon through struts during crimping.Various embodiments of a method can include fabricating a stent from aradially expanded tube.

Various embodiments of methods to fabricate stents and stent-balloonassemblies are described herein. In some embodiments, a stent is crimpedonto a delivery balloon to form a stent-balloon assembly by heat settinga stent made from a radially expanded tube. Generally, the insidesurface of a stent may be positioned adjacent the outside surface of thedelivery balloon and allowed to heat set onto the outside surface of thedelivery balloon.

When a polymer is heated, the polymer tends to shrink from a deformedstate to an originally deformed state. In the case of a stent fabricatedfrom a radially expanded tube, heating the stent can cause the stent toshrink from the expanded diameter towards the original or deformeddiameter.

Therefore, a stent may be crimped by heating the stent to a temperatureabove ambient temperature. In an embodiment, heating the stent allowsthe stent to heat shrink onto the outer surface of the delivery balloon.In one embodiment, the stent may be crimped onto the balloon with nosubstantially inward radial force on the stent. In another embodiment,the stent may be crimped both with heat shrinking and inward radialpressure.

However, heating the stent can reduce or dissipate the induced molecularorientation from radial expansion. Thus, there is concomitant loss ordissipation of induced strength due to loss in orientation. Theembodiments of the invention include fabricating a stent such that aselected degree of residual induced orientation and strength in thestent remains even after heat shrinking or crimping the stent onto theballoon.

The invention provides methods for crimping a stent to a deliveryballoon. In some embodiments, a method includes radially expanding apolymeric tube of an original inside diameter to an expanded insidediameter. In an embodiment, the method also includes positioning thestent around a delivery balloon and heating the stent to a temperatureabove ambient temperature. By heating the stent to a temperature aboveambient temperature, the stent heat shrinks onto the delivery balloon,conforming to the outer surface of the balloon to the stent.

FIG. 3( a) depicts a polymeric tube 310 radially expanded from anoriginal inside diameter D1 to a radially expanded inside diameter D2.As depicted, a polymeric tube 310 has an initial inside diameter D1.Polymeric tube 310 having initial inside diameter D1 is formed by anymeans, such as blow molding. Polymeric tube 310 is radially expanded, asindicated by arrow 315, to a radially expanded polymeric tube 320 havingradially expanded inside diameter D2. After expanding the polymerictube, a stent may be fabricated from the radially expanded polymerictube 320 using methods known to those of skill in the art, includingforming a pattern in the polymeric tube, such as with a laser. Once thestent is fabricated, the stent may then be positioned around a deliveryballoon. As depicted in FIG. 3( b), the stent is heated to a temperatureabove ambient temperature and allowed to heat shrink from expanded stent320 to a heat shrunk stent 330, as indicated by an arrow 325, havingdiameter D3.

In some embodiments, the polymeric tube from which the stent isfabricated has an original inside diameter that is less than the outsidediameter of the delivery balloon. The polymeric tube with an originalinside diameter is smaller than the delivery balloon. In this way, whenthe stent fabricated from the radially expanded tube is allowed to heatshrink to the balloon diameter, there is adequate residual radialorientation and thus residual induced strength at the crimped diameter.

In one embodiment, the polymeric tube that is later radially expandedhas an original inside diameter of from about 0.001 to 0.05, or morenarrowly, 0.008 to 0.01 inches. As known by those skilled in the art, apolymeric tube is formed by means of various types of methods,including, but not limited to extrusion, blow molding, or injectionmolding. The polymeric tube may also be formed from sheets or films thatare rolled and bonded.

The degree of radial expansion may be quantified by a blow-up ratio:Outside Diameter of Deformed Tube/Original Inside Diameter of Tube. Fromthe blow up ratio, a percent radial expansion can be determined.(Blow up ratio−1)*100=% Radial Expansion

The polymeric tube may be radially expanded to an inside diametergreater than the size of an outside diameter of the delivery balloon.For example, the polymeric tube may be radially expanded to an insidediameter 25% to 50% greater than the outside diameter of the balloon. Insome embodiments, the polymeric tube is expanded radially at least 300%,400%, 600%, or at least 900%. By radially expanding the polymeric tubeto a diameter greater than the outside diameter of the delivery balloon,adequate residual radial expansion remains after heat shrinking thestent onto the delivery balloon. In one embodiment, the inside diameterof the radially expanded polymeric tube is from about 0.060 inches toabout 0.070 inches.

In some embodiments, a stent is fabricated from the radially expandedtube. The inner surface of the stent may then be positioned over theouter surface of the balloon and allowed to heat shrink onto the balloonso to conform to the outer surface of the balloon. After heat shrinking,sufficient residual radial expansion of the stent is maintained asrequired by the stent. For example, at least 200%, 300%, or at least400% residual expansion of the heat shrunk stent may be maintained inthe stent as compared to the original inside diameter of the polymerictube. Residual expansion is the amount of radial expansion ororientation remaining in the stent after heat shrinking as compared tothe original inside diameter of the polymeric tube. Residual expansiondepends on the diameter of the original polymeric tube, the degree ofradial expansion of the polymeric tube, as well as the degree that thestent formed of the polymeric tube is heat shrunk after heating theradially expanded stent. The degree that the stent heat shrinks afterheating the stent depends on the diameter of the balloon, as the balloonacts to limit further heat shrinking when the inside diameter of thestent reaches the outside diameter of the balloon. The residual percentexpansion is defined as follows:(diameter of heat shrunk stent/original diameter of tube beforeexpansion−1)*100=% Residual Radial Expansion

As mentioned previously, the diameter of the stent after heat shrinkingdepends on the diameter of the balloon because the stent heat shrinksonto the balloon upon heating. Therefore, upon increasing a balloon'sdiameter, a tube with a larger original diameter before expansion can beused to arrive at a same % residual radial expansion. One advantage ofstarting with a larger original diameter tube is that it may be easierto manufacture than smaller diameter tubes, such as with conventionalblow molding processes.

Alternatively, if a balloon with a larger diameter is used, and the sizeof the original polymeric tube is the same, the same residual radialexpansion can be achieved even though the original polymeric tube isexpanded to a lesser degree, because the expanded tube heat shrinks to alesser degree while maintaining a certain percent radial expansion.

In one embodiment, the outside diameter of the balloon is increased toreduce the degree that the stent heat shrinks to arrive at the outersurface of the balloon. In this way, the loss of radial expansion in theprocess of heat shrinking is reduced. The extent that the stent isrequired to heat shrink and conform to the size of the outside diameterof the balloon is reduced. For example, rather than an inside diameterof the balloon of 0.033 inches, the outside diameter of the balloon canbe, for example, 0.040 inches. In this way, the stent positioned overthe larger diameter balloon heat shrinks less to conform to a largerdiameter of the balloon, and less radial expansion is lost during heatshrinking. The size of the balloon can be of any size that allowsdelivery into a selected lumen.

FIG. 4( a) depicts a side view of a balloon-catheter assembly 400.Balloon-catheter assembly 400 includes a balloon 420 attached to acatheter 410.

FIG. 4( b) depicts a side view of a stent 435 fabricated from anexpanded tube positioned over balloon 420 to form a medical assembly440. To heat shrink the stent 435 onto a delivery balloon 420, innersurface 445 of stent 435 may be positioned adjacent an outer surface 450of delivery balloon 420 and heated. Stent 435 may be heated for aboutone minute to about two hours, more narrowly, between about two minutesto about ten minutes, or more narrowly, between 30 seconds to about 1minute. Upon heating stent 435, stent 435 heat shrinks to form heatshrunk stent 445 that conforms to surface 450 of balloon 420, therebyincreasing stent retention on the balloon.

Several ways to heat the radially expanded stent and cause heatshrinking onto the balloon may be used in the invention. For example, aheated gas may be blown on the stent. Also, the stent and the balloonmay be placed within or around a heated mold. Further, the stent and theballoon may be placed in an oven. Still further, the stent may be heatedwith a heated crimper.

The stent may be heated to a temperature above ambient temperature.Also, the temperature may be between about Tg to the melting temperature(Tm) of the polymer of the stent. Further, the temperature may bebetween about Tg to about Tg+50° C. The temperature of the stent may beincreased gradually or rapidly. The temperature of the stent may beincreased and then maintained at a certain temperature. The temperatureof the stent may also be increased then decreased. In one embodiment,heat may be applied to the stent while applying inward radial pressurebefore, after, and/or during the time that the stent is heated.

FIG. 4( c) depicts a side view of a heat shrunk stent 445 on balloon420, forming a medical assembly 460. To form medical assembly 460, innersurface 445 of stent 435 may be positioned adjacent the outer surface450 of balloon 420 while stent 435 is in an expanded state. Stent 430may be heated before, during, or after being positioned around balloon420. Stent 430 then “heat shrinks” or decreases in diameter and conformsto the surface of balloon 420. In one embodiment, heat applied tomedical assembly 460 facilitates balloon protrusion 470 through stent445. Heat shrunk stent 445 conforms to outer surface 450 of balloon 420.Protrusions 470 of balloon 420 through stent 430 increase stentretention on balloon 420. Higher retention thus may be achieved byincreasing the number and/or size of protrusions 470 of balloon 420through stent 430.

Without being limited by theory, it is believed that during “heatshrinking,” the radial orientation or alignment of polymer chains inheat shrunk stent 445 are reduced due to relaxation of the polymerchains. Because the molecular orientation in the stent is relaxed duringheat shrinking, mechanical properties are also modified during heatshrinking. The degree of reduction in polymer chain alignment depends onthe temperature of the stent. For example, below the glass transitiontemperature of a polymer, polymer segments may not have sufficientenergy to move past one another. In general, relaxation of polymer chainalignment during heat shrinking may not be induced without sufficientsegmental mobility. Above Tg, heat shrinking and relaxation may bereadily induced since rotation of polymer chains, and hence segmentalmobility, is possible. Between Tg and Tm of the polymer, rotationalbarriers exist, however, the barriers are not great enough tosubstantially prevent segmental mobility. As the temperature of apolymer is increased above Tg, the energy barriers to rotation decreaseand segmental mobility of polymer chains tend to increase. As a result,as the temperature increases above Tg, heat shrinking and polymer chainrelaxation are more easily induced.

Increasing the temperature of the stent causes heat shrinking of thestent and results in reduction of circumferential polymer chainalignment and modification of the mechanical properties of the polymerin the stent. In one embodiment, the stent is heated to a temperatureabove the Tg of the polymer. As the temperature increases above Tg,segmental mobility increases, which allows increase in relaxation ofpolymer chains. Consequently, the amount of shrinking depends on thetemperature of a polymeric material.

In one embodiment, the temperature of the stent may be maintained atgreater than or equal to Tg of the polymer and less than or equal to Tmof the polymer for a selected period to time. In other embodiments, thetemperature of the stent is maintained at greater than or equal to Tg ofthe polymer and less than or equal to Tg+30° C.

In addition to the temperature, another variable to the degree of heatshrinking in the stent is the percent radial expansion of the polymertube. In one embodiment, as previously mentioned, the polymeric tubeused to fabricate the stent is radially expanded from an original insidediameter of the polymeric tube to at least 400%, at least 500%, at least600%, or at least 900% radial expansion. In this way, heat shrinking thestent onto the balloon allows the stent to have residual radialexpansion that is required of the stent during use. Although the percentradial expansion and circumferential orientation of polymer chains aredecreased in the heat shrinking process, a residual radial expansion ofat least 200%, 300%, or at least 400% is maintained on the stent thathas been allowed to heat shrink. Although some radial expansion andpolymer chain orientation is lost during heat shrinking, the residualradial expansion and radial strength in the stent is maintained.Additionally, retention of the stent on the balloon is increased due toballoon protrusions 440 through stent 430. Thus, stent retention on theballoon 420 is increased, while adequate radial expansion is maintained.

It may be advantageous to reduce heat quickly or quench the stent afterheat shrinking to reduce the loss of alignment in the stent, since lossof alignment is a time dependent process. In one embodiment, the stentis cooled to a temperature below Tg after the stent heat shrinks ontothe balloon. Cooling the stent to a temperature below Tg may facilitatemaintaining the stent's proper shape, size, and length following theheat shrinking. Upon cooling, the stent retains the length and shapeimposed by the outer surface of the balloon.

As indicated above, a stent can be formed from a tube or a sheet rolledinto a tube. A sheet or tube, for example, may be formed by variousmethods known in the art such as extrusion or injection molding. Apattern may then be formed in the polymeric tube by laser cutting orchemical etching.

Additionally, as indicated above, a stent fabricated from embodiments ofthe stent described herein can be medicated with an active agent. Amedicated stent may be fabricated by coating the surface of thepolymeric scaffolding with a polymeric carrier that includes an activeor bioactive agent or drug. An active agent or drug can also beincorporated into the polymeric scaffolding made from the blend.

Embodiments of the method described herein may be applied to balloonexpandable stents, self-expanding stents, stent grafts, andstent-grafts. In the case of a self-expanding stent, the stent can becrimped over a support, such as a catheter. The stent is used to open alumen within an organ in a mammal, maintain lumen patency, or reduce thelikelihood of narrowing of a lumen. Examples of such organs include, butare not limited to, vascular organs such as, for example, coronaryarteries or hepatic veins; renal organs such as, for example, urethrasand ureters; biliary organs such as, for example, biliary ducts;pulmonary organs such as, for example, tracheas, bronchi andbronchioles; and gastrointestinal organs such as, for example, esophagiand colons.

A stent may be configured to degrade after implantation by fabricatingthe stent either partially or completely from biodegradable polymers.Polymers can be biostable, bioabsorbable, biodegradable, or bioerodable.Biostable refers to polymers that are not biodegradable. The termsbiodegradable, bioabsorbable, and bioerodable, as well as degraded,eroded, and absorbed, are used interchangeably and refer to polymersthat are capable of being completely eroded or absorbed when exposed tobodily fluids such as blood and may be gradually absorbed and eliminatedby the body.

A biodegradable stent may remain in the body until its intended functionof, for example, maintaining vascular patency and/or drug delivery isaccomplished. For biodegradable polymers used in coating applications,after the process of degradation, erosion, absorption has beencompleted, no polymer will remain on the stent. In some embodiments,very negligible traces or residue may be left behind. The duration istypically in the range of six to twelve months, although other durationsare possible.

Biodegradation refers generally to changes in physical and chemicalproperties that occur in a polymer upon exposure to bodily fluids as ina vascular environment. The changes in properties may include a decreasein molecular weight, deterioration of mechanical properties, anddecrease in mass due to erosion or absorption. Mechanical properties maycorrespond to strength and modulus of the polymer. Deterioration of themechanical properties of the polymer decreases the ability of a stent,for example, to provide mechanical support in a vessel.

Representative examples of polymers that may be used to fabricate astent coating include, but are not limited to, poly(N-acetylglucosamine)(Chitin), Chitosan, poly(hydroxyvalerate), poly(lactide-co-glycolide),poly(hydroxybutyrate), poly(hydroxybutyrate-co-valerate),polyorthoester, polyanhydride, poly(glycolic acid), poly(glycolide),poly(L-lactic acid), poly(L-lactide), poly(D,L-lactic acid),poly(D,L-lactide), poly(caprolactone),poly(L-lactide-co-ε-caprolactone), poly(trimethylene carbonate),polyester amide, poly(glycolic acid-co-trimethylene carbonate),co-poly(ether-esters) (e.g. PEO/PLA), polyphosphazenes, biomolecules(such as fibrin, fibrinogen, cellulose, starch, collagen and hyaluronicacid), polyurethanes, silicones, polyesters, polyolefins,polyisobutylene and ethylene-alphaolefin copolymers, acrylic polymersand copolymers other than polyacrylates, vinyl halide polymers andcopolymers (such as polyvinyl chloride), polyvinyl ethers (such aspolyvinyl methyl ether), polyvinylidene halides (such as polyvinylidenechloride), polyacrylonitrile, polyvinyl ketones, polyvinyl aromatics(such as polystyrene), polyvinyl esters (such as polyvinyl acetate),acrylonitrile-styrene copolymers, ABS resins, polyamides (such as Nylon66 and polycaprolactam), polycarbonates, polyoxymethylenes, polyimides,polyethers, polyurethanes, rayon, rayon-triacetate, cellulose, celluloseacetate, cellulose butyrate, cellulose acetate butyrate, cellophane,cellulose nitrate, cellulose propionate, cellulose ethers, andcarboxymethyl cellulose. Additional representative examples of polymersthat may be especially well suited for use in fabricating a stentaccording to the methods disclosed herein include ethylene vinyl alcoholcopolymer (commonly known by the generic name EVOH or by the trade nameEVAL), poly(butyl methacrylate), poly(vinylidenefluoride-co-hexafluororpropene) (e.g., SOLEF 21508, available fromSolvay Solexis PVDF, Thorofare, N.J.), polyvinylidene fluoride(otherwise known as KYNAR, available from ATOFINA Chemicals,Philadelphia, Pa.), ethylene-vinyl acetate copolymers, and polyethyleneglycol. The stents may also be metallic; low-ferromagnetic;non-ferromagnetic; biostable polymeric; biodegradable polymeric orbiodegradable metallic.

Some embodiments of the present invention are illustrated by thefollowing Examples. The Examples are being given by way of illustrationonly and not by way of limitation.

EXAMPLES Example 1

Polymeric tube of poly(L-lactide) (PLLA) was extruded to an insidediameter of 0.008 in. Polymeric tube was expanded to an inside diameterof 0.06 in. Patterns were then cut into the tube to fabricate a stent.The stent was positioned around a balloon-catheter assembly, where theouter diameter of balloon was 0.03 inches. Stent and balloon-catheterassembly was then heated to 100° C. for 1 minute with a heated crimper.The stent was allowed to heat shrink to the outer diameter of theballoon. Heat shrunk stent had a residual radial expansion of 200%.

Example 2

Polymeric tube of poly(L-lactide) was extruded to an inside diameter of0.008 in. Polymeric tube was then expanded to an inside diameter of0.06. Patterns were then cut into the tube to fabricate a stent. Thestent was positioned around a balloon-catheter assembly. Outer diameterof balloon was 0.04 inches. Stent and balloon-catheter assembly was thenheated to 100° C. with a heated crimper. Heat shrunken stent had aradial expansion of 200%.

This invention has been described in relation to certain examples of itsapplication, such as its applicability to stents made up ofsemi-crystalline PLLA. The examples are not intended nor should they beconstrued as limiting this invention in any manner. Those skilled in theart will recognize, based on the disclosures herein, other polymers andother stents to which the invention herein may be applied. All suchpolymers and stents are within the scope of this invention.

1. A method for fabricating a medical assembly comprising: providing apolymeric tube fabricated from a polymer of an original inside diameter;radially expanding the polymeric tube; fabricating a stent from theradially expanded polymeric tube; positioning the stent around a supportelement; heating the stent, wherein the heated stent is above ambienttemperature; and allowing the heated stent to radially contract onto thesupport element, wherein the heated stent radially contracts at leastpartially due to heat shrinking of the polymer of the stent, and whereinthe inside diameter of the contracted stent is greater than the originalinside diameter of the polymeric tube.
 2. The method according to claim1, wherein the polymer is poly(L-lactide).
 3. The method according toclaim 1, wherein the support element is a delivery balloon.
 4. Themethod according to claim 1, wherein the support element is a catheter.5. The method according to claim 1, wherein the polymeric tube isradially expanded to an inside diameter greater than the outsidediameter of the support element.
 6. The method according to claim 1,wherein the polymeric tube is radially expanded to an inside diameter25% to 50% greater than the outside diameter of the support element. 7.The method according to claim 1, wherein a percent radial expansion ofthe radially expanded polymeric tube is from about 400% to about 900% ascompared to the original inside diameter of the tube.
 8. The methodaccording to claim 1, wherein the stent is heated to a temperature ofabout glass transition temperature to about glass transition temperature+50° C. of the polymer in the stent.
 9. The method according to claim 1,wherein the heated stent is allowed to radially contract to a size ofabout an outside diameter of the support element.
 10. The methodaccording to claim 1, wherein the contracted stent has a radialexpansion of about 200% to about 400% as compared to the original insidediameter of the polymeric tube.
 11. The method according to claim 1,wherein the support element has an outside diameter of about 0.040, andthe stent heat shrinks to an inside diameter of about 0.040.
 12. Themethod according to claim 1, wherein the polymer comprises abioabsorbable polymer.
 13. The method according to claim 1, whereinfabricating a stent from the radially expanded tube comprises forming apattern on the radially expanded tube including a plurality of struts.14. The method according to claim 1, further comprising applying inwardradial pressure on the stent to facilitate the radial contraction of thestent.
 15. The method according to claim 1, wherein the stent radiallycontracts onto the support element with no inward radial force on thestent.
 16. A method for fabricating a medical assembly comprising:providing a polymeric tube fabricated from a polymer of an originalinside diameter; radially expanding the polymeric tube; fabricating astent from the radially expanded polymeric tube; positioning the stentaround a support element; heating the stent; and applying inward radialpressure on the heated stent to facilitate radial contraction of thestent, wherein the inside diameter of the contracted stent is greaterthan the original inside diameter of the polymeric tube.
 17. The methodaccording to claim 16, wherein the heated stent radially contracts atleast partially due to heat shrinking of the polymer of the stent. 18.The method according to claim 16, wherein the heated stent is at atemperature from a glass transition temperature to a melting temperatureof the polymer.
 19. The method according to claim 16, wherein the heatedstent is above ambient temperature.
 20. A method for fabricating amedical assembly comprising: providing a polymeric tube fabricated froma polymer of an original inside diameter, wherein the polymer has aglass transition temperature; radially expanding the polymeric tube;fabricating a stent from the radially expanded polymeric tube;positioning the stent around a support element; heating the stent,wherein the heated stent is above ambient temperature and at atemperature from a glass transition temperature to a melting temperatureof the polymer; and allowing the heated stent to radially contract ontothe support element, wherein the heated stent radially contracts atleast partially due to heat shrinking of the polymer of the stent, andwherein the inside diameter of the contracted stent is greater than theoriginal inside diameter of the polymeric tube.